Fan theories say that King Sombra might have had an influence on Luna's transformation into Nightmare Moon. That's not my personal headcanon, but I have to admit that the idea is interesting, thus, this pic came to be.

EDIT: I've read the comics and Sombra has absolutely nothing to do with Luna's transformation. oh well.

Maybe he was alive, but like a toddler now that I think about it... But then again I'm sure that they defeated Discord before Sombra. On the other hand Sombra was alive during the time and he wasn't corrupted until after Discord was defeated... I need to do some research on this lol

Well he could have always been corrupted but only came to rule after discord was dealt with because well, how could you take over an empire that is in complete chaos? but yeah i think discord came first, sombra came second, and luna came last. Although it never really said how long discords rain lasted so he could've been very young at the time.... sorry if none of this makes sense! >-<

The emotions captured in their expressions is amazing! Sombra's expression makes me think of pity and condescension: he's as much manipulating her as he is trying to help a fellow ostracized pony of darkness, trying to get past all the near-brainwashing of Celestia ("Give them time, they will come around, dearest sister."). Luna on the other hand is trying to remain faithful/loyal, but she's unable to form a proper argument against Sombra's and she knows it.